Trolleys of this type are necessarily autonomous, they carry batteries to power them and are associated with driving means, such as guide by wire, optoelectronic guidance, trigonometric direction finding using laser or other means, reacting on the means for controlling their drive means.
In general, the displacement of an autonomous trolley is accomplished at a stable speed (of the order of 30 to 120 meters per minute), and stopping is preceded by a deceleration phase to a slow final positioning speed. This technique, associated with correct mastery, on the one hand, of the means for detecting the longitudinal position of the trolley and, on the other hand, of the drive means acting on the means for driving this trolley in translation, makes it possible to obtain a longitudinal precision of the order of plus or minus one, which is satisfactory or at least sufficient so as not to disturb a subsequent precision positioning. The transverse precision, measured perpendicularly to the direction of displacement, is clearly less accurate because it depends not only on surface imperfections of the ground, but also on the flatness of the latter. The result is that, even with a trolley having excellent constructional and functional qualities, imperfections of the ground alone bring about a transverse displacement of the trajectory of the trolley with respect to its theoretical trajectory. This displacement, called a deviation, may be of the order of several centimeters for a journey of several meters. A deviation of this value is unacceptable and must be corrected, otherwise with each change of direction, even with a precise angular rotation, it modifies the heading of the trolley and increases the imprecision of this positioning, that is to say the deviation between its actual transverse positioning and its theoretical positioning.
One solution includes acting on the guiding wheels of the trolley in order to accomplish heading corrections, subsequently checked by a positioning detector, but that leads to a continuous succession of correction and detection operations and cannot be applied to trolleys which do not comprise guiding wheels, but comprise other means allowing them to pivot in situ.